The modified Windkessel electrical analog model of the arterial system is gaining increasing attention from the medical community as a clinically useful tool for characterizing the human vasculature for the purpose of diagnosing, treating and monitoring cardiovascular disease. A number of studies of the cardiovascular system using the modified Windkessel model have been conducted, and correlations between the model parameters and normal and disease states have been identified. For instance, U.S. Pat. application Ser. No. 07/250,315, entitled "Method for Diagnosing Hypertension," now U.S. Pat. No. 5,442,571, discloses a method for utilizing the parameter C.sub.2 of the modified Windkessel model to diagnose, treat and monitor the vascular disease condition underlying hypertension.
The modified Windkessel model of the arterial system is shown in FIG. 1. In the model:
C.sub.1 =proximal arterial compliance (ml/mm Hg); PA1 C.sub.2 =distal arterial compliance (ml/mm Hg); PA1 L=inertence (mm Hg/ml/s.sup.2); PA1 P.sub.1 =proximal arterial (aortic) pressure (mm Hg); PA1 P.sub.2 =distal arterial (brachial) pressure (mm Hg); and PA1 R=peripheral resistance (dynes sec cm.sup.-5).
While the usefulness of the Windkessel model parameters for the diagnosis, treatment and monitoring of cardiovascular disease has become more apparent, they remain relatively difficult to use on a routine basis for two reasons. The first is the need to obtain a cardiac output measurement in order to determine the parameters. Conventional procedures for determining cardiac output, such as thermodilution and dye dilution, are surgically invasive, requiring catheterization of the patient. Physicians, in general, are reluctant to employ such procedures because of their cost, the discomfort and inconvenience to the patient, the risk of infection and other severe complications, and their relative level of complexity as compared to alternative noninvasive procedures. The second reason involves the difficulty with obtaining patient data for the modified Windkessel model from blood pressure waveforms, which also conventionally requires the insertion of an arterial catheter and the use of a transducer and other electronic equipment.
The present invention, as described hereinafter, provides an instrument which can noninvasively measure Windkessel parameters, or other impedance parameters which depend on cardiac output measurement, using a noninvasively obtained arterial blood pressure waveform of the patient. Accordingly, it is contemplated that the present invention will significantly facilitate widespread clinical use of the modified Windkessel model parameters, or other impedance model parameters, in the diagnosis, treatment and monitoring of cardiovascular disease. In particular, the invention allows for a quick, easy-to-use and noninvasive determination of the modified Windkessel parameters so that these parameters can be ascertained and used during routine physical examinations, and patient screening, treatment and monitoring. Given that the only existing practical and quick screening device for determining the status of the cardiovascular state is a blood pressure cuff (i.e., a sphygmomanometer) measurement, it is contemplated that the invention could provide a substantial and new diagnostic capability for physicians to use on a routine basis.